<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style type="text/css">
        * {
            margin: 0;
            padding: 0;
        }

        #wrap {
            width: 300px;
            height: 300px;
            background: deeppink;
        }

        #inner {
            /*width 默认值auto*/
            margin: 50px;
            height: 100px;
            background: pink;
        }
    </style>
</head>
<body>
<div id="wrap">
    <div id="inner">

    </div>
</div>
</body>
</html>

